Catalytic converters are well known for the removal and/or conversion of the harmful components of exhaust gases. Catalytic converters have a variety of constructions for this purpose. In one form, the converter comprises a rigid skeletal monolithic substrate on which there is a catalytic coating. The monolith substrate has a honeycomb-type structure which has a multiplicity of longitudinal channels, typically in parallel, to provide a catalytically coated body having a high surface area.
The monolithic substrate, and particularly the multiplicity of channels, can be coated with a slurry or washcoat of a catalytic and/or absorbent material, which are typically aqueous solutions containing ceramic particles, for example, alumina, ceria and zirconia particles. The particles may be catalytic without added material, and the particles may have an added catalytic function by dispersing a catalytic component, for example, a precious metal component, on the particles. When the channels of the substrate are open-ended, the carrier is referred to as a “flow through” carrier. When each channel is blocked at one end of the carrier body, with alternate channels blocked at opposite end-faces, the carrier is referred to as a wall-flow carrier (or filter).
The rigid, monolithic substrate can fabricated from ceramics and other materials. Such materials and their construction are described, for example, in U.S. Pat. Nos. 3,331,787 and 3,565,830 each of which is incorporated herein by reference in its entirety. Examples of ceramic materials include cordierite, alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia or zirconium silicate, and monolithic honeycomb substrates made from ceramic materials are extruded, dried and calcined. Alternatively, the monoliths can be fabricated from corrugated metal foil which is wrapped into a coil to form a honeycomb substrate. Examples of monolithic substrates made from metal foils are disclosed in U.S. Pat. No. 4,119,701 and U.S. Pat. No. 4,455,281. While corrugated honeycombs can be made from metal foils having holes formed or punched through the foil, the metallic foil generally has a low porosity. One limitation of honeycombs made from metal foils is that a catalyst layer cannot be tightly adhered to the metal substrate with a thin oxide layer formed thereon because of its low porosity. As a result, the catalyst layer, which is often a ceramic material applied as a washcoat, readily peels off the metal substrate due to the difference in thermal expansion between the ceramic catalyst layer and the metal substrate. Accordingly, ceramic monolithic honeycombs are generally preferred in the manufacture of catalytic converters for many applications.
There are various known methods of providing a washcoat layer on the wall surfaces of ceramic monolithic honeycomb substrates. The porosity of the walls of most commercially available ceramic substrates is generally less than 35%, and the pores have a mean pore size of less than about 30 microns. In addition, the pores of most commercially available substrates are generally not open, interconnected pores. Due to the pore size and the lack of open pores, washcoating of ceramic honeycomb substrate walls involves forming layers on the walls of the substrate, and the catalyst washcoat is generally on the exterior wall surfaces, as opposed to being disposed within the walls.
U.S. Pat. No. 5,334,570 discusses the issue of the back pressure effect of a catalytic converter on internal combustion engine performance. As is widely known, as back pressure decreases, engine performance generally improves. A decrease in back pressure is associated with an increase in the aggregate open transverse cross sectional area of the flow-through channels or cells of the washcoated, multichannel honeycomb substrate. This open transverse cross-sectional area is referred to in U.S. Pat. No. 5,334,570 as open frontal area or OFA. A phenomenon referred to as filleting, which will be described with respect to FIGS. 1 and 2, prevents decreasing the back pressure associated with the washcoated, multichannel honeycomb substrate.
FIG. 1 shows generally at 10 a monolithic substrate of generally cylindrical shape having a cylindrical outer surface 12, one end face 14 and an opposite end face, not visible in FIG. 1, which is identical to end face 14. The juncture of outer surface 12 with the opposite end face at its peripheral edge portion is indicated at 14′ in FIG. 1. Substrate 10 has a plurality of longitudinal fluid flow channels formed therein. Gas flow channels 16 are formed by channel walls 18, shown in FIG. 2. Gas flow channels 18 extend through carrier 10 from end face 14 to the opposite end face thereof, the channels being unobstructed so as to permit the flow of a fluid, e.g., a gas, longitudinally through carrier 10 via channels 16 thereof. As will be seen from FIGS. 1 and 2, channel walls 18 are so dimensioned and configured that gas flow channels 16 have a substantially regular polygonal shape. In FIG. 2, the shape of the channels 16 is shown as being square, except for fillet portions 20 which, in the illustrated embodiment, define in profile arcuate concave sections and comprise the juncture of adjacent ones of walls 18. Fillets 20 are formed by coating adhering to the corners of the channels, which reduces the cross-sectional area of the channel and decreasing the open frontal area of the substrate 10, which leads to an increase in back pressure.
As shown in FIG. 2, the width in cross section of channels 16 is indicated by W, the width in cross section of any side of the geometric square figure S superimposed on the cross sectional view of gas channel 16. Each side of the square figure S defines the nominal width W in cross section of the regular polygon approximated by the cross section profile of gas channel 16. The width W corresponds to the straight line distance extending perpendicularly from the substantially flat planar mid portion of one channel wall 18 to that of an opposite wall 18. The term “nominal width” channel walls is used to have the meaning illustrated herein, i.e., the width in cross section of one side of the polygon defined by the channel cross section profile if the filleted corners are ignored (or are nonexistent, as may be the case when the term is used with reference to structures according to embodiments of the invention). W would correspond to the actual physical width in cross section of the walls 18 if concave portions 20 were eliminated, in which case the cross section profiles would be substantially geometrically perfect squares. The arcuate surface length of fillet portions 20 is geometrically indicated in FIG. 2 by arc A, and the width in cross section of the substantially planar central portion of channel walls 18 is indicated by W′. It should be noted that the concave juncture provided by fillet portions 20 and the adjacent walls 18 could also be provided if fillet portion 20 were flat, i.e., defined in cross section a straight, rather than arcuate profile. Coating 22 of a refractory metal oxide is usually provided as a support for the catalytically promoting material. The deposition of coating 22 is indicated in dot dash lines only on the lower half of gas flow channel 16, for clarity of illustration. It will be appreciated that such coating is normally deposited substantially over the entire surfaces of each of gas channels 16 as will be shown further below.
U.S. Pat. No. 5,334,570 discusses various ways of addressing the fillet problem illustrated above. On one hand, reducing the amount of coating would reduce filleting, however, this would also reduce the amount of catalyst disposed on the channels of the catalytic converter to treat the exhaust gases flowing through the catalytic converter. One previous way to reduce filleting and to provide an adequate amount of washcoat is to form the walls of the monolithic honeycomb substrate with catalyst particles embedded in the walls, as described in U.S. Pat. Nos. 4,637,995, 4,657,880 and 4,888,317. These patents describe articles and process for co-extruding precursors of honeycombs and catalytic supports. This has been referred to as catalyst-in-wall, but as noted in U.S. Pat. No. 5,334,570, this approach has not provided catalytic activity on par with conventional catalytic converters having washcoat deposited on the wall. U.S. Pat. No. 5,334,570 observes that applying washcoats in the pores of the walls of ceramic honeycomb substrates is generally not successful in obtaining a catalytic converter that performs comparably with traditional catalytic converters having washcoat disposed on the walls. One reason for the performance deficiency is that high temperatures are required to sinter the extruded green body to produce the ceramic honeycomb and this invariably causes an irreversible loss of catalytic activity. Other techniques for adding catalytically active material to the process bodies include, for example, decomposing metal salts, as described in U.S. Pat. No. 4,522,940, in the support pores. This is widely used but differs from the traditional washcoat processes because of the use of solutions and because they do not incorporate a solid phase. The pore structure of the support material is typically less than 15 microns, preventing the transport of a solid phase throughout the support. Techniques such as synthesizing a catalytically active material, for example a zeolite, on an existing support as described by Speronello et al in U.S. Pat. No. 4,628,042 and Brown et al. in U.S. Pat. No. 4,157,375 are also undesirable since the entire ceramic honeycomb support must be subjected to the synthesis conditions. This represents a handling problem and significant cost.
A solution proposed in U.S. Pat. No. 5,334,570 is to deposit colloidal particles in the pores of the ceramic honeycomb walls. Colloidal particles are defined as particles having a size in the range of 0.001 to 0.2 microns, more particularly 0.001 to 0.1 microns, and even more specifically, in the range of 0.001 to 0.05 microns.
It is not believed that catalytic converters made in accordance with the teachings of U.S. Pat. No. 5,334,570 have been commercially successful. A possible shortcoming of using colloidal particles as defined in U.S. Pat. No. 5,334,570 is that obtaining and processing colloidal particles is not only expensive, but it is difficult to provide washcoat slurries with sufficiently high solids content to provide a catalytic converter with acceptable catalytic activity. In addition, it is difficult to obtain zeolites in colloidal form, which are typically larger in size than the size range discussed in U.S. Pat. No. 5,334,570.
Another approach is to make the honeycomb out of a catalytic material. A large catalyst “loading” could be achieved without altering the shape of the honeycomb channels. In fact, a large volume of SCR catalysts are made this way. The homogeneous product is effective because the catalyst is relatively low cost and relatively low cost extrusion technology can be used. The low extrusion costs arise because the homogenous product is seldom produced, in large commercial volumes, with cell densities greater than 100 cpsi. For applications to stationary power plants, mechanical strength is not a great concern since the honeycombs are packed into steel “cribs” that are designed to carry the mechanical loads of the surrounding catalysts. Thermal stresses are not a great concern since these large power plants heat up and cool down slowly. The very large mass of catalyst and its stationary nature, minimize stresses resulting from vibration.
Cell densities greater than 300 cpsi are possible and have been produced, but the relatively low strength of the extruded catalyst makes extruding thin walled honeycombs very difficult. In order to take advantage of the higher cell densities, the ability to create thinner walls is required. Recently, the extruded product has been successfully applied to on road truck applications at cell densities competitive with a coated product. The mechanical strength of these honeycombs is significantly poorer than the coated product requiring compromises such as limitations in frontal area of individual blocks and special packaging requirements to accommodate the lower strength product.
The limitation is not so much the technology to create thinner walls, but instead creating thinner walls with sufficient strength to prevent structural collapse of the honeycomb. One common way to improve honeycomb strength is through the use of ceramic fibers. These fibers do not form an inter-connected three dimensional network and do not by themselves constitute a free standing structure. As cell density increases, it becomes more difficult to force the fibers through the smaller die openings. Thus, the ability to produce thin walls and the lack of an interconnected skeletal network limit this technology. Other techniques such as the addition of inorganic binders can be effective but their presence can lead to changes in the porosity of the honeycomb. Generally, as inorganic binders as added to the extrusion mix the strength of the green body increases but the porosity and pore inter-connectivity is decreased. Thus, in order to achieve the honeycomb strength, a trade-off is made that reduces the effectiveness of the catalyst.
It is a continuing goal to develop a catalyst composite having sufficient washcoat loading and catalytic activity to treat exhaust gases. It would be desirable to provide catalyst composites with washcoat material disposed predominantly in the wall of the substrate, and if desired, to achieve loadings of up to 7.0 g/in3 without substantially increasing back pressure.